Automatic gain control circuits



W. R. KOCH ,AUTOMATIC GAIN CONTROL CIRCUIT .Filed May 25, 1934 Feb. 9, 1937.

INVENTOR WINFIELD R. KOCH BY/ ATTORNEY Patented Feb. 9, 1937 UNITED STATES PAT AUTOMATIC GAIN CONTROL CIRCUITS of Delaware Application May 25, 1934, Serial No. 727,376

Claims.

'My present invention relates to gain control circuits, and more particularly to automatic volume control arrangements for radio receivers.

One of the main objects of my present invention is to provide an automatic volume control arrangement for a radio receiver, and particularly one adapted for battery or direct current operation, the automatic volume control arrangement being essentially characterized by its utilization of amplified control voltages for the gain control function, and the method of producing and utilizing the control voltage consisting of the steps of producing electrical wave energy at sub-audible or super-audible frequency, and rectifying the wave energy in response to signal amplitude variations.

Another important object of the invention is to provide, in an automatic volume control system for a radio receiver, a local oscillator, the output of which is rectified, and the rectified voltage being used to bias off the amplifier tubes of the receiving system.

Another object of the present invention is to provide an automatic gain control system for a high frequency amplifier wherein the amplified signal controls the amount of bias developed by a rectifier operating to rectify the voltage output of an oscillator, and a single tube being used for the oscillation, rectification and control functions.

Still other objects of the present invention are to improve generally the efficiency and simplicity of automatic volume control systems, and more especially to provide a compact and durable type of automatic volume control arrangement which is not only reliable in operation, but economically assembled in a radio receiver.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, will best be understood by reference to the following description, taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing:-

Fig. 1 schematically shows one circuit arrangement embodying the present invention, and

Fig. 2 shows a modified form of the invention.

Referring now to the accompanying drawing, wherein like reference characters denote similar circuit elements of the two figures, there is shown in Fig. 1 a conventional type of superheterodyne receiver which comprises a radio frequency amplifier of the tunable type, a frequency changer network, conventionally represented here as including a local oscillator and a first detector, an intermediate frequency amplifier and a signal demodulator or second detector. The demodulated output of the second detector is impressed upon an audio frequency amplifier, and the output of the latter is impressed upon a reproducer. Of course, the frequency changer network may utilize a combined first detectorlocal oscillator of any well known type, and it Will be understood that the tuning means, which are usually variable condensers, of the signal input circuits of the radio frequency amplifier and the first detector, as well as the local oscillator circuits, will be mechanically uni-controlled. Such arrangements are all conventional and'well known to those skilledin this art.

The numeral I designates the usual signal collector which is coupled to the tunable signal input circuit of the radio frequency amplifier, and the signal collector in this case may comprise a. grounded antenna circuit. In order to maintain a substantially constant intensity level of signal energy fed to the second detector, there is utilized an automatic volume control system of a novel and highly efiicient type. The volume control system, which is of the type regulating the gain of one or more amplifiers of the receiving system, comprises a multi-function tube 2. This tube is of the 6F? type and comprises a common cathode 3 having associated therewith two groups of independent cold electrodes. One of these groups includes the control grid 4 and the plate 5, and the other group includes the plate 6,

as well as the grids l, 8, and 9 which are disposed between the common cathode 3 and the plate 6. The specific construction of the tube 2 may be of any well known type, and it is pointed out that such pentode-triode tubes are well known at the present time. They are essentially characterized by the fact that the common cathode 3 furnishes the electron streams, which are independent, for the two groups of cold electrodes. The plate 5 is connected to the positive terminal of a source of direct current voltage +B, (the source not being shown to preserve simplicity of disclosure), through a path which includes lead Ill and a coil II. The control grid 4 is connected to the grounded cathode 3 through a path which includes coil t2, the latter being coupled to coil II as at E3; a condenser I l being connected across coil l2 to tune it to a sub-audible or super-audible frequency. There is thus formed -efiiciency of the rectifier circuit of tube 2. this way amplified automatic Volume control with the plate 5, control grid 4 and common cathode 3 an oscillator network resonating at the frequency of the tuned circuit I4-I2. As stated, this frequency is preferably below, or above, the audible frequency range.

The plate 6 is connected to the cathode 3 through a path which includes the automatic volume control lead I5, and the grounded resistor I6 across which the control voltagedevelops; a reactive means, such as a condenser I'I, connects the plates 6 and 5 and provides a transfer path for the oscillation energy developed in the'oscillator network. The oscillation energy is rectified in the rectifying circuit which includes plate 6, resistor I6 and cathode 3, and the direct current voltage component of the rectified'oscillation energy is utilized as a gain control voltage for theradio frequency and intermediate frequency amplifier stages. This is accomplished in the well known manner by connecting the negative side of resistor I6 to the grid circuits of the radio frequency amplifier and the intermediate frequency amplifier, and a filter network I8, I9 is inserted between the A. V. C. leads to the amplifiers and the negative side of resistor I6. Of course, the signal grid of the frequency changer circuit may also have a gain control voltage impressed upon it, if desired.

The grid 8 has a positive voltage of about 90 volts impressed upon it, and the grid 9 may be .connected to the cathode 3 so as to provide a field at cathode potential between the signal grid 1 and the plate 6. The grid 1 is connected to a point between the intermediate frequency amplifier and second detector network through a condenser 20, andthe grid is maintained at a cut-off bias in the absence of impressed signals by connecting it to a source of fixed negative bias C through a resistor 2|. It is pointed out that direct current voltage sources for the various electrodes of tube 2, as well as the other tubes of the system, may be battery sources or a mains .D. C. source. The power source may also be the usual rectified alternating current source.

The operation of the automatic volume control system shown herein will be more readily understood whenit is pointed out that the oscillator network impresses the generated waves upon the diode rectifier circuit between plates 6 and cathode 3, and the efficiency of rectification is controlled by the grid I. Since the control grid I is normally biased to cut-off, in the absence of signals onthe control grid there will be no gain control voltages applied to the controlled amplifiers. In other words, these amplifiers are then operating at maximum sensitivity, and this is the condition desired for weak sig- '.nal reception, as when receiving distant stations.

When a strong local station is being received, or when the receiver is being used on an automobile, and the signal carrier fades for some reason, the production of gain control voltage is minimized because the flow of signal currents through condenser becomes less. However, as the transmission of signals through condenser 20 increases, the normal cut-off bias of grid'I is reducedby the impressed s'ignals,,an d thus the rectifier circuit becomes more efiicient and the voltage drop across resistor I6 increases. It will, therefore, be seen that the signals impressed on control grid I actually control the rectification In Voltages are secured for the gain control function, since the oscillator supplies the energy for the control voltages. The elements of tube 2 perform the oscillation, control and rectification functions.

In Fig. 2 is shown a modification of the arrangement of Fig. 1, and this modification essentially consists in utilizing a tube 2' in place of tube 2, the tube 2 being a tube of the 2A7 type wherein five grids are disposed between the cathode and the plate 3I. In this modification the oscillator network includes the electrode 32 functioning asthe oscillator anode; this electrode 32 is capacitatively coupled to plate 3I and connected to coil I I, as in the case of Fig. 1.

The oscillator grid 33 is connected to the tuned circuit I2I4, as described in connection with Fig. 1, the grids 34 and 35 being maintained in the present case, however, at a positive voltage of substantially volts. The numeral 36 designates the control grid upon which the signals are impressed, and it will be seen that this control grid is normally maintained biased to cut-off in the absence of impressed signals, as in the case of Fig. 1. Otherwise, the system shown in Fig.2 is identical with that shown in Fig. 1, and for this reason the remaining portions of the circuit need not be shown.

The modification of Fig. 2 functions in the same manner as that of Fig. 1, and the oscillations generated in the oscillator network are impressed upon the rectification network through condenser II. The rectifier circuit in this embodiment includes the cathode 30 and the plate 3|, signals impressed upon grid 36 controlling the development of control voltage across resistor I6.

The A. V. C. oscillator may operate, in either of Figs. 1 or 2, at frequencies between 5 to 10 cycles, or 20,000 to 30,000 cycles. The plate 6, or plate 3I in Fig. 2, is not always negative. During a portion of each cycle of the associated oscillator the plate becomes positive, and plate current will,

from the scope of my invention, as set forth in the appended claims.

What I claim is:-

1. In combination with a high frequency signal transmission tube, an oscillator arranged to produce electrical waves, a rectifier including at least a cathode, an anode and a control electrode therebetween, means for normally biasing the control electrode in a sense to minimize the space current flow between said anode and cathode, an impedance connected between the anode and cathode, means for impressing said waves between said anode and cathode thereby to produce a direct current voltage across the impedance by rectification of said Waves, means for impressing signal energy upon said control electrode to reduce said bias and permit said rectification, and means for impressing said direct current voltage upon said tube to reduce the gain thereof.

2. In a volume control system for a receiver electrodes cooperating to provide a. local oscil- 5 lator circuit, and certain of the electrodes cooperating to provide a rectifier circuit for locally produced oscillations from said oscillator circuit, means for impressing the local oscillations upon the rectifier circuit, means for impressing signals transmitted to said demodulator upon at least one of the electrodes of said rectifier circuit, and a direct current connection between another electrode of said rectifier circuit and again control electrode of said transmission tube.

3. In a system as defined in claim 2, said rectifier electrodes and said oscillator electrodes having a common cathode, and the electron streams of the rectifier electrodes and the oscillator electrodes being independent.

4. In a system as defined in claim 2, all of the said electrodes within said tube having a common cathode, and the electron stream from the said cathode being common to all of the said electrodes.

5. In a system as defined in claim 2, said oscillator electrodes including a plate and a grid which are inductively coupled through a tuned circuit arranged to resonate at a frequency outside the audible range, and said rectifier electrodes including a plate and at least one grid normally biased to cut-off in the absence of received signal energy.

6. A radio receiver of the superheterodyne type which includes at least one stage of radio frequency amplification preceding the mixer stage and at least one stage of intermediate frequency amplification preceding the demodulator, the receiver being characterized by the inclusion of an automatic volume control system which comprises a tube provided with a cathode, a pair of plates within said tube, a control grid disposed between one of the plates and said cathode, circuits associated with said one plate, control grid and cathode to provide an oscillator network adapted to produce oscillations at a. super-audible frequency, a second control grid disposed between the remaining plate and the cathode, reactive means for transmitting the oscillations generated in said oscillator network to the said second plate, a signal transmission path connected between an element of said receiver and the said second control grid, and a direct current connection between the second plate and at least one of said receiver amplifier stages.

'7. In a system as defined in claim 6, means for normally biasing the second control grid to cutoff in the absence of impressed signals.

8. In a system as defined in claim 6, a positive screen grid disposed between the second plate and said second control grid, and a grid at cathode potential disposed between the screen grid and the second plate.

9. A radio receiver of the superheterodyne type which includes at least one stage of radio frequency amplification preceding the mixer stage and at least one stage of intermediate frequency amplification preceding the demodulator, the receiver being characterized by the inclusion of an automatic volume control system which comprises a tube provided with a cathode, a pair of plates within said tube, a control grid disposed between one of the plates and said cathode, circuits associated with said one plate, control grid and cathode to provide an oscillator network adapted to produce oscillations at a sub-audible frequency, a second control grid disposed between the remaining plate and the cathode, reactive means for transmitting the oscillations generated in said oscillator network to the said second plate, a signal transmission path connected between an element of said receiver and the said second control grid, and a direct current connection between the second plate and at least one of said receiver amplifier stages.

10. In a receiver as defined in claim 2, means for tuning the local oscillator circuit to a frequency outside the audible frequency range.

WINFIELD R. KOCH. 

